The Ecological Significance of Giant Clams in Coral Reef Ecosystems
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Biological Conservation 181 (2015) 111–123 Contents lists available at ScienceDirect Biological Conservation journal homepage: www.elsevier.com/locate/biocon Review The ecological significance of giant clams in coral reef ecosystems ⇑ Mei Lin Neo a,b, William Eckman a, Kareen Vicentuan a,b, Serena L.-M. Teo b, Peter A. Todd a, a Experimental Marine Ecology Laboratory, Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543, Singapore b Tropical Marine Science Institute, National University of Singapore, 18 Kent Ridge Road, Singapore 119227, Singapore article info abstract Article history: Giant clams (Hippopus and Tridacna species) are thought to play various ecological roles in coral reef Received 14 May 2014 ecosystems, but most of these have not previously been quantified. Using data from the literature and Received in revised form 29 October 2014 our own studies we elucidate the ecological functions of giant clams. We show how their tissues are food Accepted 2 November 2014 for a wide array of predators and scavengers, while their discharges of live zooxanthellae, faeces, and Available online 5 December 2014 gametes are eaten by opportunistic feeders. The shells of giant clams provide substrate for colonization by epibionts, while commensal and ectoparasitic organisms live within their mantle cavities. Giant clams Keywords: increase the topographic heterogeneity of the reef, act as reservoirs of zooxanthellae (Symbiodinium spp.), Carbonate budgets and also potentially counteract eutrophication via water filtering. Finally, dense populations of giant Conservation Epibiota clams produce large quantities of calcium carbonate shell material that are eventually incorporated into Eutrophication the reef framework. Unfortunately, giant clams are under great pressure from overfishing and extirpa- Giant clams tions are likely to be detrimental to coral reefs. A greater understanding of the numerous contributions Hippopus giant clams provide will reinforce the case for their conservation. Tridacna Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Zooxanthellae (http://creativecommons.org/licenses/by-nc-nd/3.0/). Contents 1. Introduction . ...................................................................................................... 112 2. Methods . ...................................................................................................... 112 2.1. Literature survey . ......................................................................................... 112 2.2. Population estimates of ecologically relevant parameters . ...................................................... 112 3. Giant clams as food . ................................................................................... 114 3.1. Productivity and biomass ......................................................................................... 114 3.2. Food for predators and scavengers. ......................................................................... 115 3.3. Expelled materials. ......................................................................................... 115 4. Giant clams as shelter . ................................................................................... 117 4.1. Shelters for coral reef fish......................................................................................... 117 4.2. Shell surfaces for epibionts. ......................................................................... 117 4.3. Hosts for ectoparasites . ......................................................................................... 118 4.4. Hosts for commensals . ......................................................................................... 118 5. Reef-scale contributions of giant clams ................................................................................... 118 5.1. Contributors of carbonate ......................................................................................... 118 5.2. Bioeroders . ......................................................................................... 118 5.3. Topographic enhancement . ......................................................................... 118 5.4. Source of zooxanthellae (Symbiodinium spp.) ......................................................................... 119 5.5. Counteractors of eutrophication. ......................................................................... 120 6. Conclusion . ...................................................................................................... 120 ⇑ Corresponding author. Tel.: +65 65161034. E-mail addresses: [email protected] (M.L. Neo), [email protected] (P.A. Todd). http://dx.doi.org/10.1016/j.biocon.2014.11.004 0006-3207/Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 112 M.L. Neo et al. / Biological Conservation 181 (2015) 111–123 Acknowledgements . ......................................................................................... 121 Appendix A. Supplementary material . ...................................................................... 121 References . ......................................................................................................... 121 1. Introduction however, stocks are declining rapidly in many countries (bin Othman et al., 2010; Andréfouët et al., 2013) and extirpations are As recently summarized by Bridge et al. (2013, p.528) coral occurring (Kinch and Teitelbaum, 2010; Neo and Todd, 2012, reefs globally are ‘‘suffering death by a thousand cuts’’. Some of 2013). these, including global warming and ocean acidification, are There exists a substantial body of work on the biology and notorious and possibly fatal. Others, such as the loss of particular mariculture of giant clams, but their significance in the coral reef species or genera, are generally less pernicious and do not garner ecosystem is not well understood. Some previous researchers have the same attention. Of course, all reef organisms have a role to play provided anecdotal insights into their likely roles, i.e. as food, as but giant clams (Cardiidae: Tridacninae), by virtue of their sheer shelter, and as reef-builders and shapers. For example, Mercier size (Yonge, 1975), well developed symbiosis with zooxanthellae and Hamel (1996, p.113) remarked: ‘‘Tridacna face many dangers. (Yonge, 1980), and highly threatened status throughout much of They are most vulnerable early in their life cycle, when they are prey their geographic range (Lucas, 1994), perhaps deserve special con- to crabs, lobsters, wrasses, pufferfish, and eagle rays.’’ In a popular sideration. Based on fossil tridacnine taxa, these iconic inverte- science article, Mingoa-Licuanan and Gomez (2002, p.24) com- brates have been associated with corals since the late Eocene mented: ‘‘clam populations add topographic detail to the seabed (Harzhauser et al., 2008) and facies of more recent Tridacna species and serve as nurseries to various organisms... Their calcified shells are common in the upper strata of fossilized reefs (Accordi et al., are excellent substrata for sedentary organisms.’’ Finally, Hutchings 2010; Ono and Clark, 2012). Modern giant clams are only found (1986, p.245) stated: ‘‘giant clams are recognisable in early Holocene in the Indo-West Pacific (Harzhauser et al., 2008) in the area reefs and if similar densities occurred to those on recent reefs, giant bounded by southern Africa, the Red Sea, Japan, Polynesia (exclud- clams have had a considerable ongoing impact on reef morphology.’’ ing New Zealand and Hawaii), and Australia (bin Othman et al., Even though there is evidence that giant clams contribute to the 2010). There are currently 13 extant species of giant clams (see functioning of coral reefs, this has very rarely been quantified. Table 1), including two recently rediscovered: Tridacna noae (Su Cabaitan et al. (2008) represents the only study to experimentally et al., 2014; Borsa et al., 2014) and Tridacna squamosina (previously demonstrate the benefits that giant clams can have on coral reefs. known as T. costata)(Richter et al., 2008), one new species: They showed that, compared to control plots, the presence of clams Tridacna ningaloo (Penny and Willan, 2014), and an undescribed had significantly positive effects on the richness and abundance of cryptic Tridacna sp. (Huelsken et al., 2013). Tridacna maxima is fish species and various invertebrates. Here, based on existing lit- the most widespread, while Hippopus porcellanus, Tridacna mba- erature and our own observations, we examine giant clams as con- lavuana (previously known as T. tevoroa), T. ningaloo, T. noae, Trid- tributors to reef productivity, as providers of biomass to predators acna rosewateri, and T. squamosina have much more restricted and scavengers, and as nurseries and hosts for other organisms. We distributions (Rosewater, 1965; bin Othman et al., 2010; Penny also examine their reef-scale roles as calcium carbonate producers, and Willan, 2014; Su et al., 2014). Tridacna gigas is by far the largest zooxanthellae reservoirs, and counteractors of eutrophication. Our species, reaching shell lengths of over 120 cm and weights in findings lead to the conclusion that healthy populations of giant excess of 200 kg (Rosewater, 1965). Since pre-history, giant clams’ clams benefit coral reefs in ways previously underappreciated, high biomass and